A short guide to CORS Troubleshooter
Check the rule, then test the assumption.
Compare browser origin, preflight, and allow headers to find the missing CORS rule. Here is the practical way to prepare the input, read the output, and decide what to check next.
01
When to reach for it
Compares key=value request and response facts for origin, method, and credential-rule conflicts. It is most useful when you are dealing with blocked by CORS or preflight failed. Start with the smallest example that still shows the behavior.
- Look for blocked by CORS, preflight failed, credentials error.
- Reduce the example until one observation can change the result.
02
How to prepare the input
Paste a focused example in key=value request and response facts. Leave out unrelated noise so the result stays easy to verify.
- Use the accepted format: key=value request and response facts.
- Keep the facts that make the behavior reproducible.
- Do not treat missing context as a reason to invent an answer.
Example input
Safe sample
origin=https://app.example.com allowOrigin=https://api.example.com method=POST allowMethods=GET,POST credentials=true allowCredentials=false
Run this first to see the shape of the result. Then change one meaningful fact and confirm that the output changes with it.
03
How to read the result
Reports the CORS rule conflicts or consistency found in the supplied facts. Read the finding beside the evidence that produced it. The result narrows the next check; it does not claim to have changed your system.
- Separate the observed fact from the suggested next check.
- Prefer the smallest reversible experiment that can confirm or reject the finding.
- Save the output when it belongs in an incident note, review, or handoff.
04
Know the boundary
A rule finding is a focused lead, not proof that the surrounding system has no other failure.